A code-breaking book which aims to change the image of William Shakespeare and reveal him as a subversive who embedded dangerous political messages in his work is to be published in Britain.

Far from being an ambitious entertainer who played down his Catholic roots under a repressive Elizabethan regime, Shakespeare took deliberate risks each time he took up his quill, according to Clare Asquith's new book Shadowplay. She argues that the plays and poems are a network of crossword puzzle-like clues to his strong Catholic beliefs and his fears for England's future. Aside from being the first to spot this daring Shakespearean code, Asquith also claims to be the first to have cracked it.

'It has not been picked up on before because people have not had the complete context,' she explained this weekend. 'I am braced for flak, but we now know we have had the history from that period wrong for a long time because we have seen it through the eyes of the Protestant, Whig ascendancy who, after all, have written the history.'

It is now widely accepted that the era was not a period of political consensus, says Asquith. Instead, it was a time in which opposition voices were banished and censorship meant the burning of illegal pamphlets and printed works.

As a result the Catholic resistance, which had been going for 70 years by the time Shakespeare was writing, had already developed its own secret code words; a subversive communication system which the playwright developed further in his work.

'They inevitably had a hidden language, and Shakespeare used it rather like the composer Shostakovich used political codes in the 20th century,' she said.

Asquith, the wife of a British diplomat who was posted to Moscow and Kiev during the Cold War, says that while she was living in the Soviet Union she began to understand how 'dissident meanings' worked in live theatre.

Shakespeare, she claims, adopted some of the more general Catholic code terms that were current, such as the use of the words 'tempest' or 'storm' to signify England's troubles, but he also used new cyphers. Asquith argues, for example, that his obsession with the theme of romantic love was much more than a crowd pleaser.

Constancy in love was Shakespeare's way of alluding to the importance of a true faith in the 'old religion', she says. More specifically, his puns and metaphors often circled around certain key phrases. For instance, to be 'sunburned' or 'tanned', as are his heroines Viola, Imogen and Portia, was to be close to God and so understood as a true Catholic.

Already hailed as a triumph of scholarship by writer and leading Catholic thinker Piers Paul Read (who wrote: 'It is rare when a work of such painstaking scholarship is so dramatic, important and exciting to read') and by Tom Paulin (who has called Asquith 'an inspired and compelling code-breaker'), the new book also makes the startling suggestion that Shakespeare studied covertly at an Oxford University college in order to gain such a wide-ranging literary knowledge.

This suggestion goes against the work of earlier scholars who have explained away the poet's extraordinary learning as simply the product of a rigorous Elizabethan grammar school education.

'You do get new insights into his life if you look at the code,' she said. 'He must have gone up to Oxford, as many Catholics did at the time, by finding a sympathetic college, such as Hertford, but not officially signing on.' To join as an undergraduate would have meant having to forswear his religion officially, Asquith added.

By looking closely at scenes which include particularly baffling banter to the modern ear, Asquith claims to be able to prove her case. In the first scene of Much Ado About Nothing, for example, bemusing references to 6 July are used to tease the hero, Benedick. 'Mock not, mock not,' he replies, 'ere you flout old ends any further, examine your consciences'.

To Elizabethan Catholics, Asquith argues, this was a highly significant date. It was on 6 July that Henry VIII executed Sir Thomas More, his Chancellor, for refusing to acknowledge the monarch as the supreme head of the Church in England. More had become a role model for 'recusants' or dissident English Catholics.

The significance of the date was deepened for Catholics when the young Edward VI, Henry VIII's fervently Protestant son, also died on 6 July - a coincidence that was viewed as a judgment on his heretic father. 'This is why Benedick puts a stop to the banter,' says Asquith. 'His friends have gone too far. Mock not old ends, he says - the deaths of Thomas More and Edward are not a laughing matter.'

The book, set to cause controversy among experts, is full of such detailed analysis and reads, as historian Antonia Fraser has said, 'like a literary detective story.'

What's in a name? Decoding the Bard

'Words, words, words,' said Shakespeare's Hamlet. But the words used by the subversive Shakespeare in his plays and poems disguised a hidden pro-Catholic message, according to controversial new research.

Sunburn:

The sun represented divinity, and so sunburn denotes closeness to God. Shakespeare described himself as 'tanned' in Sonnet 62.

Turtle dove:

A traditional image for the apostles, used to signify those who remained faithful in the face of persecution.

Nightingale:

The story of Philomela, who was turned into a nightingale, was an image of the desecrated church and its covert protests.

Devotion to the five wounds of Christ led to patterned emblems on the banners borne against the new regime. Shakespeare uses it in the form of flowers, birthmarks or heraldic blazons as a marker of Catholicism.